45 
Sexuality of the Uredineae . 
fertilization by means of spermatia was replaced by a simpler ‘ internal ’ 
fertilization, there would no longer be any bar to the shifting of the 
reduced sexual process to another point in the life-history. The Uredineae 
would appear to be a group in which the exact point at which the reduced 
sexual process (that has replaced in phylogeny the normal one) occurs is 
not perfectly fixed ; though when the homologues of the female cells are 
present it mainly takes place in connexion with them. The fertilization, 
however, always occurs before the formation of the first spore-form after the 
sporidia, so that aecidiospores, uredospores, and teleutospores always show 
conjugate nuclei. 
It is possible that an aecidium-bearing form may be found in which the 
nuclei become conjugate before aecidium formation, so that all the fertile 
cells will be binucleate from their first origin ; such a case will only be an 
exaggeration of the condition found in Puccinia Poarum . 
Investigation of the typical aecidium confirms the view put forward 
earlier (1) that it, like the simpler aecidium (caeoma), is to be considered as 
a sorus of female reproductive organs ; in which, however, the outer layers 
of spores or spore-mother-cells have become sterilized to form the pseudo- 
peridium. 
The function of the pseudoperidium is, no doubt, protective, and its 
development in the typical aecidium is probably related to the deeper 
point of origin of that structure as compared with the sub-epidermal origin 
of the caeoma. The latter is to be considered as the more primitive type 
from which the typical aecidium has been derived. 
A study of the reduced forms without aecidia shows clearly that these 
forms also exhibit an alternation of generations. The transition from the 
gametophyte to the sporophyte is, however, obscured in these forms owing 
to the fact that it takes place in an apogamous 1 way. The exact nature 
of the apogamy is yet unknown, but it is obviously the result of the inter- 
action of undifferentiated vegetative cells or nuclei ; and is a further stage 
of reduction of the process observed in the aecidium. In the reduced 
forms also the point of the life-cycle at which it occurs is not exactly 
fixed. 
The transition from the sporophyte to the gametophyte (i. e. reduc- 
tion) is as clear in the reduced as in the other forms, for it takes place in 
the teleutospore, which is present In all forms except the very aberrant 
genus, Endophylium . 
The gametophyte stage with single nuclei may be very well marked even 
in the reduced forms, or it may be inconspicuous, as is the case in Uromyces 
Scillarum and Puccinia Adoxae, where possibly it consists of little more 
than the promycelium. This stage, however, always passes over into the 
1 The term apogamy is used for want of a better one, but it is hardly satisfactory when applied 
to cases in which there is a fusion or association of nuclei (2 a). 
